1. Field of the Invention
The present invention is directed to distance measurement technology.
2. Description of the Related Art
People undertaking construction and repair projects frequently need to measure distances. Traditional tape measures are inconvenient. They can require the use of two people in many instances. Tape measurements can lack accuracy. A user may align the tape on a slant or bend the tape when making a measurement against a fixed object.
Optical measuring systems exist for making more accurate distance measurements. However, many of these systems have drawbacks that make them undesirable to users. Some systems require the use of expensive precision components that drive the price of the measuring device beyond the purchase point of many consumers. Other systems suffer from inaccuracy due to noise and other extraneous effects.
One traditional type of system is the narrowband ranging system. This system emits one or more modulated optical signals that produce reflections on an incident target. The system captures the reflections and determines the distance to the target based on phase shifts detected in the captured reflections. These systems typically require the use of an expensive high precision receiver, such as an avalanche gain photodiode. The performance of these systems can also erode as the signal to noise ratio falls. This can be a significant drawback, because environmental conditions in the working area can provide substantial signal attenuation.
Another traditional type of system is the wideband pulsed system. This system also emits one or more optical signals that produce reflections on an incident target. The system captures the reflections and measures the round trip signal delay to obtain the distance to the target. The system determines the time difference between the time a signal pulse departs the system and the time that the system receives a reflection of the pulse. Traditional systems identify pulse departure and arrival through threshold detection—comparing the signals to a threshold level. One typical technique is half-maximum detection, which establishes a reference threshold based on the peak intensity of the signal pulses. Unfortunately, this technique does not operate well in low signal to noise ratio environments. The system has difficulty establishing a consistent detection point, because the low signal to noise ratio increases estimation errors in the measurement of signal amplitude. Challenges also arise when trying to measure time delay between signal pulses. When an asynchronous clock is employed to measure the time between pulses, significant inaccuracies can occur unless the system employs measurement intervals with impractically long durations. In order to avoid such measurement intervals, the system can employ expensive high-speed components with substantial power consumption.